Fuel cells are increasingly being used as power source for electric vehicles and other applications. An exemplary fuel cell has a membrane electrode assembly (MEA) with catalytic electrodes and a proton exchange membrane formed between the electrodes. Gas diffusion media play an important role in PEM fuel cells. Generally disposed between catalytic electrodes and flow field channels in the fuel cell, they provide reactant and product permeability, electronic conductivity, and heat conductivity, as well as mechanical strength needed for proper functioning of the fuel cell.
During operation of the fuel cell, water is generated at the cathode electrode based on electrochemical reactions involving hydrogen and oxygen occurring within the MEA. Efficient operation of a fuel cell depends on the ability to provide effective water management in the system. For example, the diffusion media prevent the electrodes from flooding (i.e., filling with water and severely restricting O2 access) by wicking product water away from the catalyst layer while maintaining reactant gas flow from the bipolar plate through to the catalyst layer.
The gas diffusion media are generally constructed of carbon fiber containing materials. Although carbon fibers are themselves relatively hydrophobic, it is usually desirable to increase the hydrophobicity or to at least treat the carbon fiber with a more stable hydrophobic coating. Adding a hydrophobic agent such as polytetrafluoroethylene (PTFE) to the carbon fiber diffusion media is a common process for increasing the hydrophobicity. This process is normally done by dipping carbon fiber papers into a solution that contains PTFE particles and other wetting agents, such as non-ionic surfactants.
Even though coating the diffusion media with PTFE generally improves cell performance, further improvement is still desirable, especially as it relates to the ability of the fuel cells to shed product water and operate at high relative humidities.